TiN is employed as a barrier material to prevent spiking of aluminum lines into a silicon substrate. TiN is generally deposited by sputtering in a PVD chamber. FIG. 1 illustrates a conventional sputtering chamber 10. A target 12, made of the material to be sputter deposited, Ti in this instance, is connected to a source of DC power 14. A substrate support electrode 16 is mounted so as to be spaced from and parallel to the target 12, and bears a substrate 18 thereon which is to be coated. A pair of magnets 20, 20' are mounted behind the target 12 and a gas inlet 22 permits one or more gases to be fed to the chamber. A gas exhaust system 24 comprising a gas outlet and a vacuum pump permits evacuation of the chamber 10 to the desired pressure, which is generally below about 20 millitorr.
During sputtering, the power is initiated and inert gas molecules, such as of argon, are fed to the chamber. When TiN is to be deposited, a mixture of argon and nitrogen gases is fed to the chamber. The argon gas molecules are attracted to the target by the DC power 14, and bombard the target 12, sputtering off particles of target material which can deposit on the substrate 18. However, since particles of target material are sputtered in all random directions, the bottom coverage of TiN films in high aspect ratio vias/contacts is very low.
In order to improve bottom coverage, a modified sputtering chamber as shown in FIG. 2 has been proposed. In FIG. 2, a chamber 170 includes a target 172, a substrate support 174 mounted spaced from and opposite to the target 172. A source of DC power 180 is connected to the target 172 and a gas inlet 197 permits entry of inert and reaction gases stored in vessels 192 and 194 respectively. Flow meters 196 and 198 monitor the gas flow rates. The chamber 170 also includes an internal helical coil 186 connected to an RF power source 188. This coil 186, when activated, forms a high density plasma region between the target 172 and the substrate support 174. Metal particles sputtered from the target 172 pass through this plasma region and become ionized. These positively charged metal ions are attracted to the negatively biased substrate support 174 (through power source 182), improving the deposition rate onto the substrate 175. In a preferred embodiment, the helical coil 186 is made of the same material as the target 172, so that any particles that become sputtered from the coil 186 will be of the same material as that of the target 172 and the deposited film will not be contaminated with other materials.
A vacuum pump 190 connected to an exhaust line 191 maintains the desired pressure in the chamber 170. Generally a pressure of about 30 millitorr is used to sputter titanium for example in the presence of nitrogen to form golden TiN.
If the vacuum pump 190, generally a cryogenic pump, is fitted with an adjustable gate valve 199, the pressure in the chamber can be adjusted by varying the speed of the pump. If the gate valve is fully open, the chamber pressure remains low, at about 10 millitorr. However, this pressure is too low to provide sufficient metal ionization in the plasma region. Thus if the pumping speed is decreased, a higher pressure can be maintained in the chamber for adequate ionization of the metal particles.
FIG. 3 illustrates a hysteresis plot of chamber pressure in millitorr versus flow rates in sccm for an argon flow rate of 25 sccm to a chamber as in FIG. 2 operated at a power of 5 kW DC and 2.5 kW RF and a substrate temperature of 200.degree. C. In order to obtain TiN under such conditions, a nitrogen flow rate of over 55 sccm is required. However, such a high nitrogen flow rate increases the chamber pressure to about 35-38 millitorr. Because of gas scattering effects at such high pressures, the sheet resistance of TiN has a poor uniformity.
FIG. 4 is a contour plot of sheet resistance of the TiN film deposited at 38 millitorr pressure. The uniformity is quite poor, about 18%, 1 sigma.
Thus it would be highly desirable to be able to deposit TiN at low pressures of about 25 millitorr and lower. In accordance with the process of the invention, the pumping speed to the chamber is increased to reduce the pressure in the chamber.